Pneumatic pressure controllers for regulating a flow of fluid between a high pressure source of supply and a regulated, lower pressure environment are well known. Such controllers monitor the regulated pressure downstream of a throttling valve and compare it to the ambient pressure for producing a modulated actuator control pressure signal. The actuator signal drives an pneumatic actuator for positioning the control valve and thereby throttling the control valve from the supply source.
One common method of achieving the modulated actuator pressure signal is by means of a control nozzle having a variable flow restrictor driven by a mechanical linkage coupled to a differential pressure diaphragm. The diaphragm compares the pressure downstream of the throttle or control valve against an ambient or other reference pressure, varying the discharge area of the control nozzle and hence, the upstream nozzle pressure.
An actuator control conduit transmits the upstream nozzle pressure to an pneumatic actuator which in turn positions the throttle valve for regulating the pressure downstream. Such controllers typically use the fluid from the high pressure supply to feed the control nozzle and position the pneumatic actuator and hence may be subject to errors introduced as the supply source pressure is increased.
One such error may arise in a controller arrangement in which this pneumatic actuator is of the "half-area" type. Such actuators include two linked, differential area pistons which are separately pressurized by the fluid supply source and the modulated upstream nozzle pressure. Modulated pressure therefore rises with increased supply pressure.
As the required modulated pressure increases, the pressure on the nozzle restrictor plate also increases contributing to a droop in actual regulated pressure downstream of the valve as the linkage is driven backward by the static pressure on the restrictor plate. This error is further compounded by the need to increase the size and flow capability of the control nozzle and control conduit to allow for the possibility of increased leakage in the pneumatic actuator which may result from the increased pressure requirement. A larger control nozzle, of course, requires a larger restrictor plate thereby further elevating the backforce on the deflector plate.
One prior art solution to this problem is to increase the area of the pressure deflectable diaphragm, however, it will be appreciated that the use of larger and, therefore, heavier components alters the dynamic response of the controller possibly reducing the initial response time due to the increased component mass, as well as altering the damping characteristics of the linkage and diaphragm such that undesirable pressure fluctuations could occur during certain operating conditions.
What is required is a pressure controller which is able to compensate for increased supply pressure and control nozzle flow without altering the mechanical configuration and size of the nozzle flow restrictor linkage in the pressure comparator.